The present invention is directed to a process and apparatus for flocculating and clarifying a solid-liquid slurry by removing solids therefrom, thereby clarifying the liquid, while simultaneously concentrating insoluble solids to a relatively dense mud bed.
More specifically, the present invention is directed to such a process and apparatus whereby it is possible to simultaneously deaerate, preflocculate and clarify such a slurry.
While the present invention is suitable for the clarification of a wide range of solid-liquid slurries, the present invention is viewed as being particularly suitable for the clarification of sugar juices and other difficult to clarify slurries the particulate matter of which is difficult to flocculate and coagulate by conventional systems. Thus, the following discussion will be with reference to the deaeration, preflocculation and clarification of sugar juices, but it is to be understood that the concept of the present invention is not limited thereby and rather is applicable to other solid-liquid slurries as will be apparent to those skilled in the art.
Over the years, the clarification of sugar juices in the sugar industry continuously has been improved. Originally, sugar juices were clarified by adjusting the pH with lime to approximately neutral, heating to boiling and allowing to settle for several hours in gravity settling tanks. This however was a batch operation and was not continuous. The liming and heating increase the amount of insoluble solids and typically allow coagulation and precipitation thereof during extended settling of up to three to four hours. Disadvantages of this system are, in addition to the long required settling time and the lack of continuous operation, that the extended settling time allows acidification of the juice, and the varied quality of the sugar juice due to differences in geography, time, weather, etc. made clarification relatively unreliable and frequently result in a substandard product.
Improvement in equipment designs allowed continuous clarification of sugar juices by clarifiers with multiple trays allowing mud to settle on the individual trays, thereby incorporating several clarifiers into a single vessel. Nevertheless, the basic problems of extended clarification times of two to four hours remain, as do the problems associated with the wide variety of sugar juice quality.
With the advent of chemical polymers such as polyacrylamides it became possible to treat juices with small quantities of such polymers, thus accomplishing some flocculation and coagulation of juices and thereby improving the reliability of clarification in multi-tray clarifiers. Capacity also was increased, but the improvements were relatively marginal, with settling times rarely if ever being less than two hours.
The next development in the industry involved rapid clarifiers such as the Eis clarifier and the SRI (Sugar Research Institute) clarifier, wherein juices that had been limed and treated with a chemical polymer were introduced into a clarification vessel in such a manner that they entered below the mud level therein and flocculated particles were formed within the mud bed. The liquor or liquid was forced to rise through the mud bed, achieving a degree of filtration, and such liquid then was removed from an upper clarified liquid level. Such clarifiers offer rapid clarification with retention times on the order of 30 minutes. However, the operating sensitivity and the variation of sugar juice quality internationally have restricted general acceptance of such clarifiers. Another development in the industry involves a preflocculating apparatus, such as disclosed in U.S. Pat. Nos. 3,963,513 and 4,135,946. In this type of apparatus, sugar juices are limed, deaerated and pretreated with polymer in a preflocculating chamber wherein the juices are literally rolled back onto themselves with the continuing application of fresh polymer, such that a "snowballing" effect causes a preflocculation of solids in the slurry. These flocs of solids are uniform in size and of greater density than in previous flocculation procedures. The thus preflocculated juice passed on to conventional multi-tray clarifiers exhibits qualities of more rapid settling and superior clarified juice purity and clarity. Further, such procedure requires less lime for pH control, this resulting from reduced acidification of the juice consistent with shorter settling times in the clarifier. Laboratory tests prove that such preflocculated juice will settle within minutes and when passed through conventional multi-tray clarifiers, capacity improvements are typically 20 to 40%, i.e. a reduction in required settling time from two to three hours to one to two hours. However, a marked disadvantage of preflocculation of slurries in equipment physically separate from the clarifier is the disruption and breakage of flocs of solids during transit from the preflocculating apparatus to the clarifier. Despite the best attention to laminar flow and minimum circuits, such disruption takes place inevitably and results in loss of efficiency during clarification.